linux-kernel - Re: [PATCH v3 1/6] driver core: allow stopping deferred probe after init

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Date:   Sat, 30 Jun 2018 01:25:36 +0300
From:   Andy Shevchenko <andy.shevchenko@...il.com>
To:     Rob Herring <robh@...nel.org>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linus Walleij <linus.walleij@...aro.org>,
        Alexander Graf <agraf@...e.de>,
        Bjorn Andersson <bjorn.andersson@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Kevin Hilman <khilman@...nel.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Joerg Roedel <joro@...tes.org>,
        Robin Murphy <robin.murphy@....com>,
        Mark Brown <broonie@...nel.org>,
        Frank Rowand <frowand.list@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        devicetree <devicetree@...r.kernel.org>,
        boot-architecture@...ts.linaro.org,
        linux-arm Mailing List <linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v3 1/6] driver core: allow stopping deferred probe after init

On Thu, Jun 28, 2018 at 11:43 PM, Rob Herring <robh@...nel.org> wrote:
> Deferred probe will currently wait forever on dependent devices to probe,
> but sometimes a driver will never exist. It's also not always critical for
> a driver to exist. Platforms can rely on default configuration from the
> bootloader or reset defaults for things such as pinctrl and power domains.
> This is often the case with initial platform support until various drivers
> get enabled. There's at least 2 scenarios where deferred probe can render
> a platform broken. Both involve using a DT which has more devices and
> dependencies than the kernel supports. The 1st case is a driver may be
> disabled in the kernel config. The 2nd case is the kernel version may
> simply not have the dependent driver. This can happen if using a newer DT
> (provided by firmware perhaps) with a stable kernel version. Deferred
> probe issues can be difficult to debug especially if the console has
> dependencies or userspace fails to boot to a shell.
>
> There are also cases like IOMMUs where only built-in drivers are
> supported, so deferring probe after initcalls is not needed. The IOMMU
> subsystem implemented its own mechanism to handle this using OF_DECLARE
> linker sections.
>
> This commit adds makes ending deferred probe conditional on initcalls
> being completed or a debug timeout. Subsystems or drivers may opt-in by
> calling driver_deferred_probe_check_init_done() instead of
> unconditionally returning -EPROBE_DEFER. They may use additional
> information from DT or kernel's config to decide whether to continue to
> defer probe or not.
>
> The timeout mechanism is intended for debug purposes and WARNs loudly.

> The remaining deferred probe pending list will also be dumped after the
> timeout. Not that this timeout won't work for the console which needs
> to be enabled before userspace starts. However, if the console's
> dependencies are resolved, then the kernel log will be printed (as
> opposed to no output).

There is another patch flying around with debugfs node to dump a list
of deferred probe queue.
I dunno if it makes sense to dump it here and there and if yes, some
unification in output, perhaps?

> +       deferred_probe_timeout = simple_strtol(str, NULL, 0);

Hmm... I don't think 16-base or 8-base values are useful to support.
One subtle difference that people usually consider timeout values as
10-base and if at some point someone makes it as 0100 (no matter why),
it would be much less than expected.
08 wouldn't parsed at all.

> +       if (deferred_probe_timeout > 0) {

Would it be harmful / useful if we skip this check and run the work immediately?

> +               schedule_delayed_work(&deferred_probe_timeout_work,
> +                       deferred_probe_timeout * HZ);
> +       }


-- 
With Best Regards,
Andy Shevchenko